We plan to dissect the time course of murine leukemia virus (MuLV)-induced preleukemic changes and leukemic transformation in AKR mice. We will study a system which we have developed whereby intrathymic injection of cloned isolates of dualtropic (MCF) MuLV allows the events of leukemogenesis in AKR mice to be synchronized. Susceptible thymocytes will be identified. Virus infection will be monitored by expression of MuLV-coded cell surface antigens and by infectious center assays. We will attempt to determine whether MCF virus-infected thymocytes which show amplified expression of MuLV antigens are in an altered state of proliferation and committed to transformation. The role of virus-receptor binding will be assessed in terms of infection and transformation of thymocytes. Malignant transformation will be correlated with changes in cell size and expression of specific viral and cellular antigens. Leukemias induced by cloned virus isolates will be examined for phenotypic heterogeneity. An attempt will be made to detect activation of cellular transforming genes as part of a multi-stage model of leukemogenesis. This project has been designed to bring together several specific probes of the events of AKR leukemogenesis. Two classes of genetically marked, cloned MCF viruses will be studied which are thymotropic but which differ in leukemogenicity. A panel of conventional antisera and monoclonal antibodies against MuLV-coded antigens and against mouse alloantigens will be used to discriminate between different endogenous AKR viruses and to identify thymocyte subpopulations. A fluorescence-activated cell sorter will provide the primary means of analysis for this work, a sensitive method of quantitating immunofluorescence of fractionated thymocytes.